Unit-cell array optical signal processor

ABSTRACT

This invention provides a versatile unit cell as well as programmable and reconfigurable optical signal processors (such as optical-domain RF filters) that are constructed from arrays of those unit cells interconnected by optical waveguides. Each unit cell comprises an optical microdisk, an optical phase shifter, and at least one input/output optical waveguide, wherein the microdisk and the phase shifter are both optically connected to a common waveguide.

RELATED APPLICATIONS

Some embodiments of the present invention relate to embodiments andfeatures described in U.S. Patent App. No. 61/028,625 (Ng et al.), filedFeb. 14, 2008, the disclosure of which is hereby incorporated byreference in its entirety, and which assignee is the same as theassignee of this patent.

FIELD OF THE INVENTION

The present invention generally relates to communication andsignal-processing systems, and relates more specifically to methods anddevices for optical signal processing.

BACKGROUND OF THE INVENTION

The ever-increasing demand for broadband communication systems has ledto optical-transmission systems based on optical waveguides such asoptical fibers and on optical processing elements for use in thesesystems. Generally, in high-performance communication systems, photonscontinue to supplant electrons as messengers.

Significant effort has been spent towards optical integrated circuitshaving high complexity and advanced functionality. As is described inDriessen et al., Proc. of SPIE Vol. 5956, 2005, which is herebyincorporated by reference herein, optical microresonators can beconsidered as promising building blocks for filtering, amplification,switching, and sensing. Active functions can be obtained by monolithicintegration or by a hybrid approach using materials with thermo-optic,electro-optic, and optoelectronic properties and materials with opticalgain.

In a common configuration in microresonator-based sensors, amicroresonator is placed in close proximity to an optical waveguide suchas an optical fiber whose geometry has been specifically tailored-forexample, tapered or etched to a size of 1-5 microns. The taperingmodifications to the waveguide result in there being a substantialoptical field outside the waveguide, so that light can couple into themicroresonator and excite its eigenmodes. These eigenmodes may be ofvarious types, depending upon the resonant cavity geometry.

Optical microdisks or microring waveguides have been used in the past asboth resonators and switches. Djordjev et al. describe a microdisk thatcan work as a tunable filter as well as a path switch for light of aparticular wavelength (IEEE Photonics Technology Letters, vol. 14, no.6, June 2002, pp. 828-830). The switching occurs when the resonancefrequency of the microdisk resonator matches the frequency of the lightin an input waveguide, thereby coupling light from that waveguidethrough the resonator into an output waveguide. When the resonancefrequency is tuned to be different than the frequency of the waveguidedlight, that light is not coupled into the resonator but rather remainsin the input waveguide. This switching behavior occurs for light whosewavelength or frequency is within a very narrow range of values.

An optical ring resonator whose switching behavior is controlled solelyby controlling the coupling that occurs at its tworesonator-to-waveguide coupling regions is described by Yariv(Electronics Letters, vol. 36, no. 4, February 2000, pp. 321-322).According to Yariv, the amount of light that remains in a firstwaveguide or that is coupled via the resonator into a second waveguideis controlled by adjusting the coupling coefficients of the two couplingregions-between first waveguide and resonator, and between secondwaveguide and resonator.

When the resonator is operated near its “critical coupling” point, theattenuation of the light in the resonator after a round trip isapproximately equal to the amount of light coupled at the couplingregion. At this critical coupling point, there can be perfectdestructive (phase) interference at the output waveguide segment of afirst waveguide between the light transmitted from the input waveguidesegment of that first waveguide and the light coupled from the resonatorinto that output waveguide segment. When this perfect destructiveinterference occurs, all of the light is coupled into the resonator.That light, coupled into the resonator from the first waveguide, can becoupled almost completely out of the resonator through a secondwaveguide.

One type of filter known in the art is a cascade of 2×2 (two inputs andtwo outputs) finite impulse response (FIR) filtering stages such asdescribed by K. Takiguchi et al., Journal of Lightwave Technology, vol.13, no. 1, January 1995, pp. 73-82. Each FIR stage contains an opticalwaveguide 2×2 coupler that divides the light into a pair of waveguidepaths and then another optical waveguide 2×2 coupler that combines thelight from those two paths. Thus, each FIR stage can resemble aMach-Zehnder interferometer, having, in general, two arms of unequallengths.

Another type of filter is a cascade of 2×2 infinite impulse response(IIR) filtering stages such as described by K. Jinguji, Journal ofLightwave Technology, vol. 14, no. 8, August 1996, pp. 1882-1898. Thisfiltering stage is similar to the FIR stage but has an optical ringresonator coupled to one of the waveguide arms. There can be an opticalphase shifter in the other waveguide arm.

Examples of delay-line filter designs that contain both ring resonatorsand phase shifters are described by K. Jinguji and M. Oguma, Journal ofLightwave Technology, vol. 18, no. 2, February 2000, pp. 252-259 and byC. Madsen, Journal of Lightwave Technology, vol. 18, no. 6, June 2000,pp. 860-868. Both the optical ring resonator and the optical phaseshifter can be located in the same arm of a Mach-Zehnder interferometer.

Optoelectronic devices that are based on a combination of dielectricoptical waveguides (such as silica waveguides) and thin portions ofsemiconductor optoelectronic materials (such as InP or GaAs) aredescribed in U.S. Pat. No. 6,852,566 (D. Yap) and U.S. Pat. No.6,875,985 (D. Yap), both of which are hereby incorporated by referenceherein. Devices described by these patents comprise at least onedielectric optical waveguide and a layer of “active” semiconductormaterial physically bonded to the dielectric waveguide material, whereinthe “active” semiconductor material is able to generate light; detectlight; amplify light; or modulate the intensity, phase, or polarizationof the light.

In practice, both for convenience and for economic reasons, it would bebeneficial if a simple unit cell could be suitable for the constructionof filter building blocks comprising combinations of multiple unitcells. A preferred elemental unit cell would be versatile and could beused to construct the types of filters described by Jinguji et al. andMadsen, cited above, and other signal processors of various complexitiesand functionalities.

In view of the state in the art, there is a need for the aforementionedsimple unit cell, along with methods to make and use such a unit cell ina signal processor or filter. Further, there is a need in the art for aprogrammable optical microdisk capable of operating as a switch, as acoupler, and as a resonator element in a delay-line filter. Finally,there is a need to integrate an optical microdisk with an optical phaseshifter both coupled to the same optical waveguide.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, aswill now be summarized and further described in detail below.

In a first aspect of the present invention, a unit cell is providedwhich comprises an optical microdisk, an optical phase shifter, and atleast two input/output optical waveguides, wherein the microdisk and thephase shifter are both optically connected to a first input/outputoptical waveguide, and wherein the microdisk is optically connected to asecond input/output optical waveguide.

In some embodiments of the unit cell, the optical microdisk and opticalphase shifter are separately programmable. The optical microdisk cancomprise two programmable coupling regions and a programmable resonancefrequency or phase shift.

In certain preferred embodiments, the unit cell comprises a thin layerof semiconductor material in optical connection with an input/outputoptical waveguide (preferably at least two input/output opticalwaveguides). The layer of semiconductor material preferably includes anundoped region and a doped region, preferably configured substantiallylaterally. In some embodiments, the input/output optical waveguide(s)includes a core region, wherein the undoped region is located verticallyabove the core region and the doped region is not adjacent to the coreregion.

The combination of the undoped region and the doped region can form alateral P-i-N diode. In some embodiments, application of a voltage tothe P-i-N diode is capable of producing an effective change in theoptical refractive index of the undoped region in the P-i-N diode. Insome embodiments, application of a current to the P-i-N diode is capableof producing an effective change in the optical refractive index of theundoped region in the P-i-N diode.

In some embodiments, the combination of the undoped region and the dopedregion forms a lateral N-i-N device. Application of a voltage to thelateral N-i-N device is preferably capable of producing an electricalcurrent suitable to change the optical refractive index of the undopedregion in the lateral N-i-N device.

In some variations of the invention, the unit cell further comprises twoor more input optical ports and two or more output optical ports foroptically connecting to other unit cells, wherein a first pair of inputand output optical ports comprise terminations of the first input/outputoptical waveguide, and wherein a second pair of input and output opticalports comprise terminations of the second input/output opticalwaveguide.

In some variations of the invention, the unit cell comprises two or moreinput optical ports and two or more output optical ports for opticallyconnecting to other unit cells. In certain embodiments, the unit cellincludes exactly two input optical ports and two output optical portsfor optically connecting to other unit cells. The unit cell can includethree ports optically connected to the microdisk, with a fourth portoptically connected to a phase shifter.

In some embodiments, the microdisk can include an optional resistiveheating element. The microdisk can also include one or more couplingregions and one or more phase-shifting sections. In certain embodiments,the unit cell comprises two coupling regions and two phase shifters onthe microdisk, and further includes a separate phase shifter.

Another aspect of the present invention described herein provides anoptical signal processor comprising a plurality of unit cells. Theseunit cells are preferably as described herein above and in the detaileddescription.

In some embodiments, the optical signal processor is an optical filtercomprising a plurality of unit cells, wherein each unit cell includes anoptical microdisk, an optical phase shifter, and a first input/outputoptical waveguide, and a second input/output optical waveguide, whereinthe microdisk and the phase shifter are both optically connected to thefirst input/output optical waveguide (i.e., the microdisk and phaseshifter are connected to the same waveguide), and wherein the secondinput/output optical waveguide is optically connected to the microdisk.

In certain preferred embodiments of this optical filter, each unit cellcomprises a thin layer of semiconductor material, in optical connectionwith an input/output optical waveguide, at least one undoped region, andat least one doped region.

An optical filter can be a programmable delay-line filter constructed byinterconnecting, with optical waveguides, an arrayed plurality of theunit cells. This interconnection can be two-dimensional in nature. Incertain embodiments, all of the unit cells are substantially identicalin composition. In other embodiments, different unit cells havedifferent composition.

In various optical filters provided herein, some or all of the unitcells can be used as 2×2 optical couplers, 2×1 optical combiners, 1×2optical power splitters, recursive delay lines, optical path switches,optical phase shifters, or some combination of these functions.

Another aspect of the invention provides a programmablesignal-processing building block comprising a combination of four unitcells each as described herein above, wherein the building block has upto four optical inputs and up to four optical outputs. This programmablesignal-processing building block can be programmed in a manner suitablefor a function selected from the group consisting of finite impulseresponse filter, infinite impulse response filter, delay line withadjustable phase shift, optical power splitter, optical power coupler,optical switch, and recursive optical delay line.

Yet another aspect of the present invention relates to methods ofconstructing an optical signal processor comprising a plurality of unitcells. In certain embodiments, these methods comprise the common stepsin various order:

(i) providing (e.g., fabricating) a plurality of unit cells, whereineach unit cell includes an optical microdisk including a circular path,an optical phase shifter, a first input/output optical waveguide, and asecond input/output optical waveguide, wherein the microdisk and thephase shifter are optically connected by means of the first input/outputoptical waveguide, and wherein the second input/output optical waveguideis optically connected to the microdisk; and

(ii) interconnecting at least some of the plurality of unit cells with aplurality of optical waveguides, the plurality of optical waveguidesbeing connected to the unit cells by means of the first and secondinput/output optical waveguides in the unit cells.

In some embodiments of these methods, a further step is provided,comprising providing a thin layer of semiconductor material, having anundoped region and a doped region, in optical connection with theinput/output optical waveguide, in substantially each unit cell.

In some embodiments, methods further comprise programming at least someof the unit cells to function as 2×2 optical couplers each comprisingtwo inputs and two outputs, wherein the optical microdisk of the unitcells includes two coupling regions, the two coupling regions suitablyprogrammed to establish the relative amount of power coupled betweeneach of the first and second input/output waveguides and the circularpath.

In some embodiments, methods further comprise programming at least someof the unit cells to function as 2×1 optical power combiners, whereinthe optical microdisk of each of the unit cells includes first andsecond coupling regions, the first coupling region suitably programmedto partially couple light from the first input/output optical waveguideinto the circular path of the microdisk, and the second coupling regionsuitably programmed to fully couple light from the circular path to thesecond input/output optical waveguide.

In other embodiments, methods further comprise programming at least someof the unit cells to function as optical path switches, wherein theoptical microdisk of the unit cells includes two coupling regions, thetwo coupling regions both suitably programmed to either (a) fully couplelight between the first and second input/output optical waveguide andthe circular path of the microdisk or to (b) not couple light betweenthe first and second input/output optical waveguide and the circularpath of the microdisk.

In various embodiments that provide a thin layer of semiconductormaterial having an undoped region and a doped region in opticalconnection with at least one input/output optical waveguide, programmingcan be implemented by appropriately electrically energizing the undopedand doped regions of the semiconductor material.

Still another aspect of the invention provides for methods of using theunit cells and optical signal processors described herein. In somevariations, methods for optical signal processing comprise:

(i) providing at least one input optical signal;

(ii) providing a plurality of unit cells, wherein each unit cellincludes an optical microdisk, an optical phase shifter, at least oneinput/output optical waveguide, the optical microdisk and optical phaseshifter each comprising a thin layer of semiconductor material, eachfurther comprising at least one undoped region and at least one dopedregion in optical connection with the at least one input/output opticalwaveguide;

(iii) optionally programming the plurality of unit cells, comprisingapplying suitable voltages to electrical contacts of the microdisk andthe phase shifter;

(iv) directing the at least one input optical signal to the plurality ofunit cells; and

(v) using at least one output optical signal.

Certain methods of use comprise using at least some of the unit cells ina manner suitable for a function selected from the group consisting of2×2 optical coupler, 2×1 optical combiner, 1×2 optical power splitter,recursive delay line, optical path switch, and optical phase shifter.

Some methods of use comprise one or more signal-processing functionsselected from the group consisting of finite impulse response filter,infinite impulse response filter, delay line with adjustable phaseshift, optical power splitter, optical power coupler, optical switch,and recursive optical delay line.

Some methods of use can propagate information in anoptical-telecommunications network. Certain methods of use equalizeinformation in a wavelength-multiplexed network or allow for selectionof wavelength channels. Other methods of use allow for preprocessing ofRF-modulated optical signals before conversion back to the RF domain.

These and other aspects, variations, and embodiments of the presentinvention will become apparent by reference to the detailed descriptionbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an embodiment of the unit cell of theinvention.

FIG. 2 is a cross-sectional illustration of the coupling region of amicrodisk according to some embodiments.

FIG. 3 is a depiction of a second embodiment of the unit cell of theinvention.

FIG. 4 is a cross-sectional illustration of an optical phase shifter ofthe unit cell according to some embodiments.

FIG. 5 summarizes an exemplary process for fabricating a microdisk andoptical phase shifter of a unit cell, in certain embodiments.

FIG. 6 illustrates several different functional devices that can beachieved by programming the control parameters of a common unit cell.

FIG. 7 depicts a versatile building block realized by an interconnectionof four unit cells, according to some embodiments.

FIG. 8 is a certain embodiment of the filter structure of FIG. 7,operating as a 2×2 FIR building block.

FIG. 9 is a certain embodiment of the filter structure of FIG. 7,operating as a 2×2 IIR building block.

FIG. 10 illustrates an exemplary programmable filter obtained by atwo-dimensional interconnection of the versatile building block of FIG.7 used as 2×2 building-block filters.

FIG. 11 depicts an exemplary programmable filter obtained by atwo-dimensional interconnection of the versatile building block of FIG.7 used as 4×4 building-block filters.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The apparatus and methods of the present invention will now be describedin detail by reference to various non-limiting embodiments of theinvention.

Unless otherwise indicated, all numbers expressing dimensions,frequencies, parameters, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Without limiting the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques.

The disclosure herein describes, inter alia, an elemental unit cell fromwhich to construct filter building blocks. As will be appreciated by askilled artisan, the unit cell of the present invention can be aversatile component that is capable of performing the differentfunctions of the various separate components contained in filterbuilding blocks. In various embodiments, the unit cell provided by thepresent invention can operate as an optical phase modulator, an opticalresonator, or as a series combination of phase shifter and opticalresonator.

Some embodiments of the present invention are premised on therealization, at least in part, that a programmable phase shifter can beimplemented to have no net phase delay-as if the phase shifter wereabsent. Some embodiments of the present invention are also premised onthe recognition, at least in part, that a programmable microdisk can beprogrammed in a manner such that no light is coupled into that microdiskfrom its input waveguide-as if the microdisk were not present.Furthermore, some embodiments of the present invention make use of therealization that a programmable microdisk having a pair of input andoutput waveguides cascaded with a programmable optical phase shifter canperform at least some of the functions of a cascade of two programmable2×2 optical direction couplers.

Certain preferred embodiments of the present invention will now bedescribed in more detail, by reference to the accompanying figures. Thefigures are understood to provide representative illustration of theinvention and are not limiting in their content. Figures are notnecessarily drawn to scale. It will be understood by one of ordinaryskill in the art that the scope of the invention extends beyond thespecific embodiments depicted.

Reference can first be made to FIG. 1, which is a schematic of a unitcell of the invention in some embodiments. The unit cell 100 accordingto FIG. 1 comprises a programmable microdisk 110, a programmable phaseshifter 120, and optical waveguides 130 and 160 (which can also bedescribed as sections of optical waveguides). The unit cell of FIG. 1contains four input/output (I/O) ports 132, 134, 162, and 164, withtypically two of these optical ports acting as inputs and two of theseoptical ports acting as outputs. An input optical port and outputoptical port together are terminations of an input/output opticalwaveguides. For example, input port 132 and output port 134 areterminations of optical waveguide 136.

The microdisk 110 has a circular path 170 through which lightpropagates. When the light makes multiple round trips around thecircular path, the microdisk is an optical ring resonator. The microdiskhas a pair of I/O waveguides (which can be considered a first waveguide130 and a second waveguide 160). The microdisk also has at least twooptical-coupling regions 140, 150 at which light can be coupled betweenthe I/O waveguides 130, 160 and the circular path 170. Thus, light inthe microdisk travels in its circular path until that light is absorbed(or lost to scattering) or that light is coupled out from the microdiskto an I/O waveguide by means of these coupling regions. Also, lightpropagating in an I/O waveguide can be coupled into the circular path bymeans of these coupling regions. The operation of a coupling region issimilar to that of an optical-waveguide directional coupler.

The amount of light that is coupled at a coupling region can becontrolled electrically. A coupling region 140, 150 is illustrated inmore detail in FIG. 2. Each coupling region 140, 150 contains somesemiconductor material whose optical refractive index can be controlledelectrically and some means to change that optical refractive index. Forexample, the semiconductor material can be made of InP or GaAs, whichare merely exemplary materials. The semiconductor material in theoptical-coupling region 210 of coupling region 140, 150 preferably isundoped.

As used in the present disclosure, an “undoped” semiconductor is a puresemiconductor without any significant dopant species present. An undopedsemiconductor can also be referred to as an “intrinsic” or “i-type”semiconductor. For present purposes, an unintentionally dopedsemiconductor shall also fall within the meaning of an undopedsemiconductor, notwithstanding the possible imbalance between the numberof electrons and the number of holes due to the presence of anunintentional dopant which may be, for example, an impurity or anerroneously added species.

The microdisk of FIG. 1 and FIG. 2 also contains a pair of metalelectrodes 220, 240 and a pair of doped semiconductor regions 230, 250near each optical-coupling region 210. An electrical voltage appliedacross a pair of electrodes 220, 240 or an electrical current appliedthrough those electrodes can affect the optical refractive index of thesemiconductor material in the optical coupling region 210. An appliedvoltage changes the optical refractive index by means of theelectro-optic property or the electro-refractive property of thesemiconductor material. An applied current, which injects electricalcharge carriers through the semiconductor material of the couplingregion, changes the optical refractive index by means of thefree-carrier (or plasma) effect.

The light in a microdisk that operates as a resonator can make manyround trips through the circular path 170. The number of round tripsthat light makes before the light is attenuated is described by aproperty called the “intrinsic Q” of the resonator. In general, it ispreferable to minimize the attenuation of the light traveling in aresonator (maximize the intrinsic Q). The number of round trips thatlight makes before the light is coupled out through a coupling region isdescribed by a parameter called the “extrinsic Q” or “loaded Q” of theresonator. The resonator together with its input/output waveguide actslike a recursive delay line, with the effective delay being related tothe number of round trips the light coupled into a resonator makeswithin that resonator before that light is coupled out again. The amountof coupling that occurs in a coupling region (described by the couplingcoefficient of that coupling region) provides a means to adjust orprogram properties of the unit cell.

Light can be coupled into a microdisk (such as the microdisk shown inFIG. 1) through either one of its two I/O waveguides 130, 160. Likewise,light can be coupled out from a microdisk through either one of its twoI/O waveguides 130, 160. Thus, a microdisk and its pair of I/Owaveguides acts like a 2×2 optical delay device. The two portions of theI/O waveguide pair that can provide light into the microdisk can beconsidered the input waveguide segments (e.g. 136, 166). Also, the twoportions of the I/O waveguide pair that can extract light from themicrodisk can be considered the output waveguide segments (e.g. 138,168). The ends of these four segments thus constitute four optical portsof a microdisk device: two input ports (e.g. 132, 162) and two outputports (e.g. 134, 164).

It is noted that the roles of the input and output ports and the inputand output waveguide segments can be reversed, depending on thedirection in which the light travels through the circular path. Forexample, the numerical designations provided herein for the variousports and waveguide segments are consistent with light traveling in aclockwise direction around the circular path.

The time that it takes for the light to make a round trip through thecircular path of a microdisk is associated with a resonance frequency ofthat microdisk. That resonance frequency is equal to the inverse of theround trip time. The resonance frequency of a microdisk can be adjustedelectrically.

One method to adjust the resonance frequency is to heat thesemiconductor material, since the optical refractive index of thematerial is affected by its temperature. The embodiment shown in FIG. 1includes a resistive heater 190 comprising a pattern of thin metalstrips for heating the semiconductor material. The resistive heater mayprimarily heat only some portions of the circular path 170. A microdiskcan have its resonance frequency programmed and also the couplingcoefficients at its two coupling regions 140, 150 programmed. Thischaracteristic allows the microdisk to function as an optical switch oras a switched optical delay element.

A preferred unit cell also contains a separate optical phase shifter120. That phase shifter 120 can be a dielectric optical waveguide thatincludes some semiconductor material such as InP or GaAs. The opticalrefractive index of this semiconductor material can be changed by meansof an applied voltage (which establishes an electric field in thatmaterial) or by means of an applied current (which injects electricalcharge carriers through the material). A change in the refractive indexof the semiconductor material results in a change in the optical phaseof the light that has propagated through the optical phase shifter. Thephase shifter is preferably optically connected to one of theinput/output waveguides (e.g., 130) of the microdisk 110. Thus, theoptical phase of the light coupled to or from the microdisk can beprogrammed by the electrical voltage or current applied to the phaseshifter. The semiconductor material of the phase shifter preferably, butnot necessarily, is the same as the material of the microdisk.

FIG. 3 depicts another embodiment of the unit cell of this invention.The unit cell of this embodiment, like the unit cell of the firstembodiment described above in FIG. 1, comprises a programmable microdisk310, at least one programmable phase shifter 320, and sections ofoptical waveguide 330, 360. The programmable phase shifter 320 of FIG. 3is identical to the programmable phase shifter 120 of FIG. 1, in theseembodiments. The programmable microdisk of FIG. 3 is similar to theprogrammable microdisk of FIG. 1.

The microdisk of FIG. 3 has at least one phase-shifting section 380, 390in addition to the two coupling sections 340, 350. The opticalrefractive index of the semiconductor material in these phase-shiftingsections can be changed by applying an electrical voltage or current tothe sections. Each phase-shifting section 380, 390 of the resonator issimilar to the coupling regions 340, 350. Apart from the waveguide core260, which is absent from a phase-shifting section, FIG. 2 also canillustrate the parts of a phase-shifting section 380, 390. Aphase-shifting section preferably contains a pair of metal electrodes220, 240 and a pair of doped semiconductor regions 230, 250 near theportion of the circular path passing through that section. Although thephase-shifting section occupies only a portion of the overall circularpath, it is the overall round-trip time of the light that determines theresonance frequency of the microdisk. Thus, the resonance frequency canbe changed by means of the electrical voltage or current applied to thephase shifting sections.

FIG. 2 shows the cross-section of a microdisk provided by the presentinvention. This cross section is taken through one of the two opticalcoupling sections 140, 160, 340, 360 of the microdisk or through aphase-shifting section 380, 390. The microdisk is essentially a thinpiece of semiconductor material 200 that lies above a dielectricsubstrate 205 in which an optical waveguide is formed. The dielectricsubstrate 205 can comprise materials such as silicon dioxide or siliconnitride. The optical waveguide has a core region 260 of a material thathas a higher optical refractive index than the index of the remainingdielectric material, which constitutes the waveguide cladding 270.

For example, the core region 260 can be made of a material such asgermanium-doped silicon dioxide. Typical dimensions of the core regionare 2-8 μm in the vertical direction and 2-8 μm in the lateraldirection, although other dimensions are certainly possible. Awaveguide-core of Ge-doped silica, surrounded by silica cladding, ispreferred because it is known to be characterized by a low waveguideloss (per length basis). The level of Ge doping can be about 0.5-3%,such as that employed in commercial devices, or some other level of Gedoping. Other dielectric waveguides can be used, including for examplesilicon nitride (Si_(x)N_(y)) or silicon oxynitride (SiON), among otherpossibilities as will be appreciated.

A waveguided optical mode 290 can be carried in the waveguide. Thatoptical mode 290 overlaps the core region 260 and can extend somewhatbeyond the core region into the surrounding dielectric material, asshown in FIG. 2. The core region 260 is separated from the semiconductormaterial 200 by a region of the substrate material, or some othermaterial having lower refractive index than the core region. Thethickness of this separation region 275 is typically between 0.5-6 μm.

The semiconductor layer 200 is preferably thin, in comparison, and canhave a stepped profile. The portion of the semiconductor layer that isdirectly above the waveguide core region is thicker, typically having athickness of about 0.1-0.3 μm. The portion of the semiconductor layerthat is further toward the center of the microdisk can have that samethickness, about 0.1-0.3 μm, in some embodiments. The portion of thesemiconductor layer that is further toward the edge of the microdisk canbe thinner, such as less than 0.1 μm.

The semiconductor material 200, such as InP or GaAs, has an opticalrefractive index that is higher than the refractive index of thedielectric material of the substrate 205. For example, the refractiveindex of the semiconductor material can be greater than 1.5, preferablygreater than 2.0, and more preferably greater than 3.0 when SiO₂ isemployed as the dielectric substrate material (the refractive index ofSiO₂ is about 1.46). Typically, the semiconductor material is exposed tothe air (or vacuum), which has a refractive index of approximately 1.0(exactly unity for perfect vacuum).

In some embodiments, the step 202 in the semiconductor layer 200 resultsin a large change in the effective refractive index experienced by thelight. This refractive index step between the central portion of themicrodisk structure and the edge portion of that structure can beeffective for supporting a whispering gallery optical mode (a travelingwave confined close to the surface) that propagates around the microdiskstructure in a circular path. When the semiconductor layer is thin, asis preferable, much of the whispering gallery optical mode 280 does notoverlap the optically lossy semiconductor material but rather extendsoutside it, into the dielectric material or into the air, as illustratedin the FIG. 2.

The portion of the semiconductor layer 200 that overlaps with thewhispering gallery optical mode 280 preferably consists of undopedmaterial, thereby reducing the concentration of free electrical carriersin that material. Free carriers result in absorption of the light, soundoped material has lower optical attenuation. The other portions ofthe semiconductor layer, both toward the center of the microdiskstructure and toward the perimeter of the microdisk structure,preferably comprise doped semiconductor material. For example, theperimeter portion 250 could be doped with an n-type dopant. The centralportion 230 could be doped with either an n-type dopant or a p-typedopant. The doped regions can be formed by known microelectronic-devicefabrication techniques, such as ion implantation or diffusion. The dopedregions preferably do not extend through the entire circumference of themicrodisk structure but rather exist only as isolated regions. Thoseisolated regions correspond to the optical-coupling regions and thephase-shifting regions of the microdisk, as illustrated in FIG. 1 andFIG. 3.

With continuing reference to FIG. 2, metal contacts 220, 240 arepreferably formed over the tops of the doped regions 230, 250. Thesemetal contacts can be connected, by means of metal interconnect patterns(not shown), to additional metal contact pads (not shown) that arelocated somewhere on the dielectric substrate surface. Wires can beattached to those contact pads and also can connect to externalelectrical circuits that supply the voltage or current.

As an example, the central doped region 230 can be p-type and theperimeter doped region 250 can be n-type. This forms a P-i-N electricaldiode. When a reverse bias voltage is applied to this diode, an electricfield is established across the region of undoped semiconductormaterial. This electric field changes the optical refractive index ofthe material as a result of the electro-optic effect. Application of alarger bias voltage results in a larger electric field and acorresponding larger change in the refractive index. The whisperinggallery optical mode 280 responds to this change in refractive index,and the propagation velocity through that portion of the microdiskstructure is changed.

In an optical-coupling region 210, the whispering gallery optical mode280 and the waveguided optical mode 290 are preferably both present.Both of these optical modes have associated with them an effectivepropagation constant (or effective mode index). The thickness of thesemiconductor layer can be chosen such that the effective propagationconstant of the whispering gallery optical mode is approximately thesame as the effective propagation constant of the waveguided opticalmode. The thickness of the dielectric material 275 between the waveguidecore region 260 and the semiconductor layer 200 can be chosen such thatthe whispering gallery optical mode and the waveguide optical modepartially overlap each other.

In some embodiments, light power can be coupled effectively between thewhispering gallery optical mode 280 and the waveguided optical mode 290because these two modes overlap each other and because these two modeshave approximately the same propagation constant. The amount of lightpower that is coupled will depend on the length of that interaction,which relates closely to the length of that optical coupling region.However, when the refractive index of the semiconductor layer ischanged, as a result of the applied voltage or current, for example, theeffective propagation constant of the whispering gallery optical mode ischanged. The two optical modes would then not have the same propagationconstant and, thus, the effectiveness of their light power exchangewould be expected to be reduced.

If the refractive index of the semiconductor layer is increased, thepropagation constant is reduced and light will tend to be coupled intothe whispering gallery optical mode. Conversely, if the refractive indexof the semiconductor layer is reduced, light will tend to be coupledaway from the whispering gallery optical mode. Thus, the amount of lightthat is coupled into (or out from) a microdisk and out from (or into)its I/O waveguide can be controlled by means of the applied voltage orcurrent. In some embodiments, it is desirable to set the length of thecoupling region to optimally transfer the light from the waveguide intothe microdisk (or conversely from the microdisk into the waveguide) whenzero voltage or current is applied. An applied voltage or current canthen be conveniently used to modulate this transfer of the light.

FIG. 4 shows the cross section of an optical phase shifter 120, 320 thatcould be present in some unit-cell embodiments. The phase shifter hereconsists of an optical waveguide that has a higher-refractive-indexdielectric core region 460 surrounded by the lower-refractive-indexmaterial 470 of the dielectric substrate 205. The dimensions of theoptical waveguide of the phase shifter are generally (but notnecessarily) identical to those of the optical waveguide of themicrodisk. A thin layer of semiconductor material 400 lies above theoptical waveguide. The thickness of this semiconductor layer typicallyis less than 0.1 μm.

This semiconductor layer in FIG. 4 has a central portion 410, locateddirectly over the waveguide core region 460, which is undoped. Thesemiconductor layer also has other portions 430, 450, located on eitherside of the undoped region, that are doped. One of those doped regions(e.g. 450) is preferably doped n-type and the other of those dopedregions (e.g. 430) can be doped n-type or p-type. The waveguided opticalmode 490 in the phase shifter can have a substantially elliptical shapeand extend over the waveguide core region 460 as well as into thesemiconductor layer 400, in some embodiments.

Like the microdisk, metal contacts 420, 440 are formed over the tops ofthe doped regions 430, 450 in the optical phase shifter. These metalcontacts 420, 440 can be connected, by means of metal interconnectpatterns, to additional metal contact pads (not shown) that are locatedsomewhere on the dielectric substrate surface. Wires can be attached tothose contact pads and also connect to external electrical circuits thatsupply the voltage or current. As an example, one doped region can bep-type and the other doped region can be n-type (a P-i-N electricaldiode). When a reverse bias voltage is applied to this diode, anelectric field is established across the region of undoped semiconductormaterial. This electric field changes the optical refractive index ofthe material, and allows for a net change of the optical phase of thelight that has passed through the phase shifter.

As another example, the central doped region 230 of a microdisk can ben-type and the perimeter doped region 250 can also be n-type (an N-i-Nelectrical-current injector). When an electrical current is supplied tothis device, electrical charge carriers are injected from a doped regioninto the undoped region. The additional electrical carriers injectedinto the undoped region act to decrease the optical refractive index ofthat undoped semiconductor material, because of the free-carrier orplasma effect. This reduction of the optical refractive index changesthe propagation velocity of the whispering gallery optical mode 280 inthat region. Similarly, when the N-i-N structure is formed in an opticalphase shifter, application of an electrical current to that structurecan change the optical phase of the light that has propagated throughthat phase shifter.

In some embodiments, a P-i-N structure also can be used to injectelectrical charge carriers into an undoped region, by applying a forwardbias voltage to the P-i-N diode.

The optical phase shifter preferably has a central portion (as seen froma top view shown in FIGS. 1 and 3) and two end portions. The crosssection shown in FIG. 4 is of the central portion. In the end portions,the width of the semiconductor layer preferably is reduced gradually,with the width of the doped regions being reduced first and then thewidth of the undoped region also being reduced, until there is nosemiconductor layer. This tapering of the semiconductor layer can beeffective in gradually coupling the light upward to overlap thesemiconductor layer and then downward back to the dielectric waveguide.The tapered configuration can be desirable for reducing the opticalscattering loss that might occur at an abrupt transition betweensections of passive optical waveguide and the active phase shifter.

FIG. 5 depicts an exemplary process that can be used to fabricate a unitcell according to the invention. Such a process also can be used tofabricate a programmable filter or signal processor that comprisesmultiple unit cells.

In some embodiments, the process begins with the procurement orfabrication of a wafer 510 of dielectric waveguides, such as silica onsilicon waveguides, and a wafer 520 of semiconductor epitaxial material.As is known, both of these wafers can be purchased commercially (frome.g. Applied Materials, California, U.S.). The dielectric waveguides canbe made to have some desired physical pattern, such as those shown insubsequent drawings that describe exemplary optical-filterconfigurations. The dielectric waveguide wafer can be lapped andpolished to ensure that it has a very flat surface and to achieve aspecific thickness for the layer 275, 475, 575 of dielectric substratematerial that is above the waveguide core regions 260, 460, 560.

On the semiconductor substrate, a layer 502 of stop-etch material isgrown, and then the undoped semiconductor layer 200, 400, 500 used inthe optical phase modulator and the microdisk is grown. Epitaxial growthis an effective technique to obtain a precise thickness for each of thesemiconductor layers. The two wafers 510 and 520 are then wafer bonded,top-face to top-face, using known techniques for wafer bonding of asemiconductor material such as InP or GaAs to an oxide such as silicondioxide. Next, the substrate 540 of the semiconductor epitaxial wafer isremoved, generally by known lapping or grinding processes and then byknown chemical etching processes. The stop-etch layer 502 serves tocontrol the removal of the substrate 540 material and to protect thedesired undoped semiconductor layer 500. The stop-etch layer 502 is thenremoved by selective etchants to expose the undoped semiconductor layer500 that resides above and is bonded to the dielectric waveguide wafer510.

A subsequent processing step, in preferred embodiments, is to etch thepedestal 202 in the semiconductor layer 200 of the microdisk structure.This processing step also preferably thins the semiconductor layer ofthe optical phase shifter. Next, the rest of the semiconductor layer inetched away where it is not needed (and also to define the tapers in thephase shifter). Well-known photolithography and etching techniques canbe used for these processing steps. The regions 230, 250, 430, 450 ofdoped semiconductor are then formed by ion-implantation, diffusion, orby some other technique.

Then, patterns of metal ohmic contacts 220, 240, 420, 440 can be formedover the doped regions. Contact pads also are formed for the externalelectrical connections such as wire bonds. These contact pads can beformed on the regions of exposed dielectric surface that are not usedfor the optical waveguides. Finally, metal interconnect lines can beformed by known means to interconnect the ohmic contact with the contactpads.

The programmable unit cell can be a versatile component. FIG. 6illustrates several different functional devices that can be achieved byprogramming the control parameters of the same unit cell. The unit cellcan act as a power splitter (FIG. 6 a) with programmable splittingratio; the unit cell can completely bypass the microdisk (FIG. 6 b); theunit cell can bypass the separate phase shifter (FIG. 6 c); and/or theunit cell can act as a recursive delay line (FIG. 6 d) with one or twooutputs whose relative phase can be controlled. The microdisk in therecursive delay line acts as a resonator.

Optical delay-line filters can be obtained by constructing a set ofmultiple unit cells that are optically connected by optical waveguides.The interconnecting optical waveguides are essentially extensions of theinput/output waveguides or waveguide sections of the optical phaseshifter and the microdisk. In designing optical delay-line filters, itcan be convenient to use an intermediate construct or building block andthen to cascade those building blocks to form a lattice structure.

One common example of a building block is an asymmetric Mach-Zehnderinterferometer whose two arms have differing lengths. The asymmetricMach-Zehnder interferometer is a typical building block for a FIRfilter. Other common building blocks add a ring resonator to one arm, orto both arms, of a Mach-Zehnder interferometer. These building blocksare typically used to form IIR filters.

FIG. 7 shows an exemplary, versatile building block that is obtained bycombining four unit cells 701, 702, 703, 704. This building block isessentially a four-cell signal processor and has two primary inputoptical waveguide sections 710, 720 and two primary output opticalwaveguide sections 730, 740. This four-cell signal processor canfunction as a 2×2 filter structure. One unit cell 701 is used as aprogrammable 2×2 input optical coupler or switch. Another unit cell 702is used as a programmable 2×2 output optical coupler or switch. When theunit cells 701, 702 are used as an optical coupler, this building blockfilter has a Mach-Zehnder interferometer configuration comprising theinput and output couplers and two optical paths 750, 760 (or arms)between those couplers. The other two unit cells 703, 704 are placed onein each of the two optical paths 750, 760. There is, thus, aprogrammable optical microdisk in each of the two arms of theMach-Zehnder interferometer, in the embodiment described by FIG. 7.There also is a programmable optical phase shifter in at least one ofthese two arms.

Continuing with FIG. 7, the microdisk 711 in the input coupler 701 canbe programmed to function as a 2×2 optical power splitter, with variablepower splitting ratio, or as a 2×2 optical path switch. The length ofthe optical waveguide section 768 connecting unit cell 701 to unit cell704, the length of the optical waveguide section 758 connecting unitcell 701 to unit cell 703, and the optical phase shifter 721 in thatunit cell 701 can be used to establish the relative optical phase delaybetween the inputs of the two interferometer paths 750, 760. When a unitcell is used as an optical path switch, both coupling regions of themicrodisk in that unit cell are operated either in a fully uncoupledstate, with little light transferred between the I/O waveguide and thecircular path; or in a fully coupled state, with maximal transfer oflight between the I/O waveguide and the circular path.

In an alternative programming of the unit cells of FIGS. 1-4, themicrodisk 712 in the output coupler 702 can be programmed to function asa 2×2 optical power combiner, with variable power combining ratio, or asa 2×2 optical path switch. The length of the optical waveguide section769 connecting unit cell 702 to unit cell 704, the length of the opticalwaveguide section 759 connecting unit cell 702 to unit cell 703, and theoptical phase shifter 722 in that unit cell 702 can be used to establishthe relative optical phase delay between the outputs of the twointerferometer paths 750, 760. When a unit cell is used as an opticalpath switch, both coupling regions of the microdisk in that unit cellare operated either in a fully uncoupled state, with little lighttransferred between the I/O waveguide and the circular path; or in afully coupled state, with maximal transfer of light between the I/Owaveguide and the circular path.

The microdisks 713, 714 in the two arms of the programmable buildingblock of FIG. 7 can be operated as ring resonators. In such anembodiment, the building block is a fully reconfigurable opticaldelay-line filter that can have two poles and two zeroes in itscharacteristic mathematical function. The locations of these poles andzeroes and the coefficients of the characteristic function can bechanged by adjusting the coupling coefficients of the coupling regions744, 753 between the two microdisks 713, 714 and the two interferometerarms 750, 760 (which includes input/output waveguides coupled to thesemicrodisks 713, 714); the resonance frequency of those microdisks 713,714 (by means of the resistive heater or phase-shifting sections inthese microdisks); the phase shift produced by the optical phaseshifters 724, 722; and the coupling coefficients of the coupling regionsin 741, 751, 742, 752 of the microdisks 711, 712 that are used as the2×2 input and output couplers.

FIG. 8 illustrates a building block operating as a FIR filter stage. Theunit cells at the input and the output act like the 2×2 optical couplersof a Mach-Zehnder interferometer. The unit cells 703, 704 on the twoarms of the interferometer bypass their microdisks 713, 714. By havingthe light travel paths of differing length before that light reaches thefinal coupling region of the output unit cell 702, this interferometeris made asymmetric, as is typically desired. The difference betweenthose path lengths is the time-delay increment for this filter stage.These filter stages can be cascaded together to form a lattice structurefor higher-order filtering response. Each interferometer arm has avariable optical phase shifter, and the coupling ratios of the input andoutput couplers can be varied.

FIG. 9 illustrates a simple IIR filter that has a ring resonator on oneinterferometer arm and a phase shifter on the other interferometer arm.Such filters can be cascaded to form a lattice structure havinghigh-order filtering characteristics. Filter design techniques, such asthose described in U.S. Patent App. No. 61/028,625 (which has beenincorporated by reference herein above), can be used to select parametervalues for a given desired filter characteristic.

For this example, coupling region 753 in the microdisk 713 of unit cell703 is programmed to couple light between the input/output waveguide 763and microdisk 713, with microdisk 713 acting as an optical resonator. Onthe other hand, coupling region 744 in the microdisk 714 of unit cell704 is programmed such that light is not coupled between input/outputwaveguide 734 and microdisk 714.

The building block of FIG. 7 also contains two secondary input waveguidesections 770, 780 and two secondary output waveguide sections 810, 820.These secondary waveguide sections can be used in some filter designsand are not used in other filter designs, such as those illustrated inFIGS. 8 and 9. When these secondary waveguide sections are not used, theoptical coupling regions 743, 754 corresponding to those waveguides canbe programmed to suppress the coupling of light between the microdiskstructure 713, 714 and their waveguides 733, 764. This can beaccomplished, for example, by applying a sufficiently large voltageacross those coupling regions 743, 754 such that the coupling issuppressed because the propagation constants of the whispering galleryoptical mode and the waveguided optical mode are sufficiently different.

In some embodiments, the optical phase in the two arms of theinterferometer can be controlled and matched within a period of theoptical wavelength. Thus, a coherent delay-line filter can be achieved.However, the lengths of the two optical paths represented by those twoarms are not usually equal. Other building blocks can be designed tohave time-delay-matched optical paths, if that is desired for a certainapplication.

FIG. 10 illustrates, as an example, part of a programmable filter thatis constructed from a combination of multiple four-unit-cell buildingblocks similar to those shown in FIG. 7. This filter is atwo-dimensional construction or array of unit cells. The filter canfunction, for example, as a transversal filter, with multiple paralleldelay-line paths. A transversal filter configuration can be achieved byoperating the unit cell 911 at the input of the leftmost four-cellbuilding block 910 as an optical power splitter. If the other three unitcells of this building block are programmed to bypass the microdisks,this building block acts like a 1×2 power splitter. Power splitters withmore branches or taps can be obtained by cascading building block 910 tobuilding block 920 and 930, and by also operating the building blocks as1×2 power splitters, thereby forming a branching network of such 1×2power splitters. An alternative transversal filter configuration can beachieved by operating the unit cell 982 at the output of the rightmostfour-cell building block 980 in FIG. 7 as an optical power combiner. Ifthe other three unit cells of this building block are programmed tobypass the microdisks, this building block acts like a 2×1 powercombiner. Power combiners with more branches or taps can be obtained bycascading building block 980 to building block 950 and 960, and by alsooperating the building blocks as 2×1 power combiners, thereby forming abranching network of such 2×1 power combiners.

The programmable filter according to FIG. 10 also can function as alattice filter, with multiple cascaded, selectable delay-line paths. Alattice filter can be achieved by routing the optical signal from onebuilding block stage to another in cascade manner, with the input andoutput couplers of each building block stage acting like opticaldirectional couplers.

Since the input and output couplers of each building block are fullyprogrammable, those couplers also can function as optical path switches.Thus, they can be used to direct the light into a particular buildingblock filter or to divert the light away from another building blockfilter. This capability is useful when a building block contains afaulty unit cell. That faulty building block can then be bypassed.

The filter of FIG. 10 makes use of only the primary input and outputwaveguide sections of the building block of FIG. 7. When secondary inputand output waveguide sections also are present and utilized, the filtercan have additional capability. For example, each unit cell can beprogrammed to function as an optical switch, an optical coupler, or anoptical resonator.

FIG. 11 shows part of an exemplary filter that contains nested delaypaths. In this example, the light can be routed (by a microdiskfunctioning as a switch or coupler) into a “figure-eight” shapedwaveguide delay path 920. The light also can be delayed with the samemicrodisk operating as an optical resonator. Note that the figure-eightdelay path can be designed to return the light back into the samemicrodisk. Thus, the same microdisk (e.g. 990) can control two nestedresonators, or recursive optical-delay lines, that have different roundtrip times. One resonator loop can be the circular path (e.g. 970) inthat microdisk (e.g. 990) and the other resonator loop can be thefigure-eight delay path (e.g. 940) coupled to that microdisk.

Such combinations of different round trip times are useful forconstructing Vernier-effect filters. Vernier effect filters can achievenarrow passbands and yet very large frequency separation betweensuccessive occurrences of the passbands.

Various uses and applications of certain embodiments of the presentinvention will now be further described. It will be understood by one ofordinary skill in the art that the scope of the invention extends beyondthe specific uses described herein.

Some embodiments of the present invention make use of an array ofprogrammable unit cells to accomplish optical-domain signal processing.The optical signal processor can be constructed by a regular array ofversatile unit cells whose function can be changed by adjusting thevalues of its control parameters. When the unit cell does not includeany memory capability, the electrical voltages or currents for controlparameters must be supplied continually. However, in some embodiments,as will be recognized by a person of skill in this art, electronicmemory capability can be added to the electronic circuit that suppliesthe voltages and currents in order to program and control theoptoelectronic unit cell.

Several exemplary two-dimensional arrangements and interconnections ofunit cells have been described that can perform a large variety offiltering functions. With a combination of multiple unit cells of thisinvention, it is possible to achieve FIR filters, IIR filters, andcombinations of FIR and IIR filters. An optical FIR delay-line filterhas no return path in the optical circuit and its impulse response islimited, being finite in time. An optical IIR delay-line filter has oneor more return paths in its optical circuit and its impulse responsecontinues to infinite time.

According to the present invention, filters of high order (having manypoles and zeroes in their mathematical description) can be achieved byincluding more unit cells and interconnecting those unit cells withoptical waveguides. Despite the unit cell of the invention beingsimple-comprising one microdisk, one phase modulator, and at least oneI/O waveguide-filters constructed from a combination of multiple unitcells can be quite complicated. Also, different filtering functions canbe achieved simply by reprogramming a prescribed combination andinterconnection arrangement of multiple unit cells each having the samephysical construction.

Filters that can be constructed include Chebyshev filters, Butterworthfilters, and elliptic filters, having various frequency-responsecharacteristics, and reconfigurable filters, with real-time changeablefrequency-response characteristics.

In various embodiments described above or otherwise described andenabled by the invention as claimed, two-dimensional arrangementsprovide capabilities that are not possible with one-dimensional(cascade) arrangements or with branching or tapped (transversal)arrangements that have multiple parallel delay lines. For example, thesame (or substantially the same) two-dimensional arrangement of unitcells of the present invention can be programmed to achieve a FIRtransversal filter form, a FIR lattice filter form, or an IIR filterform. Also, 4-unit-cell building blocks can be programmed as 1-to-2 or1-to-4 optical power splitters or as 2-to-1 or 4-to-1 optical combinersor couplers.

Additional capabilities include the provision of bypass paths forrouting the optical signal around faulty unit cells or filter blocks,the capability to perform both serial and parallel processing, and theability to have nested delay-line processes (such as used inVernier-effect filters).

It will be evident to a skilled artisan from the examples (as describedin the drawings and accompanying text above) that a variety of opticaldelay-line filters can be obtained by configuring multiple unit cellsinterconnected by optical waveguide extensions of the input and outputwaveguide sections of those unit cells. As illustrated by the examplesof FIGS. 10 and 11, it is possible to achieve very different filtertypes using the same physical arrangement of unit cells. Opticaldelay-line filters are merely one example of signal processors that canbe provided by this invention.

One may choose to construct the filters by means of building blockfilters, such as a 4^(th)-order building block provided above as anexample (see FIGS. 10 and 11). Conversely, one may choose to design afilter by implementing the unit cell as the basic construction element.In making the filters, one may choose to make all of the microdisks havethe same physical size. One also may choose to have different microdiskshave different physical sizes.

The programmable, unit-cell-based filters described herein haveapplicability in optical telecommunications, as equalizers forwavelength multiplexed networks, or to select specific wavelengthchannels. Filters of this invention also have applicability in radiofrequency (RF) systems that involve optical fiber for distancing orelectrically isolating an RF sensor (such as an antenna) from theelectronic signal processor. The optical-domain filters of the presentinvention can be used to preprocess RF-modulated optical signals beforethose signals are converted back into the RF domain.

In this detailed description, reference has been made to multipleembodiments and to the accompanying drawings in which is shown by way ofillustration specific exemplary embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that modifications to the various disclosed embodimentsmay be made by a skilled artisan.

The embodiments, examples, variations, and figures described aboveshould provide an indication of the utility and versatility of thepresent invention. Of course, many more devices can be developed thatinvolve combinations of unit cells as provided herein. Other embodimentsthat do not provide all of the features and advantages set forth hereinmay also be utilized, without departing from the spirit and scope of thepresent invention. Such modifications and variations are considered tobe within the scope of the invention defined by the appended claims.

1. A unit cell comprising an optical microdisk, a separate optical phaseshifter, and at least two input/output optical waveguides, wherein saidmicrodisk and said phase shifter are both optically connected to a firstinput/output optical waveguide, and wherein said microdisk is opticallyconnected to a second input/output optical waveguide.
 2. The unit cellof claim 1, wherein said optical microdisk and said optical phaseshifter are separately programmable, and wherein said optical microdiskcomprises two programmable coupling regions and a programmable resonancefrequency.
 3. The unit cell of claim 1, wherein said unit cell comprisesa thin layer of semiconductor material in optical connection with saidat least two input/output optical waveguides.
 4. The unit cell of claim3, wherein said layer of semiconductor material includes an undopedregion and a doped region, wherein said undoped region and said dopedregion are configured substantially laterally.
 5. The unit cell of claim4, wherein said at least one input/output optical waveguide includes acore region, and wherein said undoped region is located vertically abovesaid core region and said doped region is not adjacent to said coreregion.
 6. The unit cell of claim 4, wherein the combination of saidundoped region and said doped region forms a lateral P-i-N diode, andwherein application of a voltage or current to said P-i-N diode iscapable of producing a change in the optical refractive index of theundoped region in said P-i-N diode.
 7. The unit cell of claim 4, whereinthe combination of said undoped region and said doped region forms alateral N-i-N device, and wherein application of a voltage to saidlateral N-i-N device is capable of producing an electrical currentsuitable to change the optical refractive index of the undoped region insaid lateral N-i-N device.
 8. The unit cell of claim 1, furthercomprising two or more input optical ports and two or more outputoptical ports for optically connecting to other unit cells, wherein afirst pair of input and output optical ports comprise terminations ofsaid first input/output optical waveguide, and wherein a second pair ofinput and output optical ports comprise terminations of said secondinput/output optical waveguide.
 9. The unit cell of claim 1, whereinsaid microdisk includes a phase-shifting section.
 10. The unit cell ofclaim 1, wherein said microdisk further comprises two coupling regionsand two phase shifters on said microdisk.
 11. The unit cell of claim 1,wherein said microdisk includes a heating element.
 12. An optical signalprocessor comprising a plurality of unit cells each described accordingto claim 1, said unit cells arranged in an array pattern, and saidsignal processor further comprising a plurality of optical waveguides,wherein each optical waveguide of said plurality of optical waveguidesinterconnects two unit cells of said plurality of unit cells.
 13. Anoptical filter comprising a plurality of unit cells, wherein each unitcell includes an optical microdisk, a separate optical phase shifter, afirst input/output optical waveguide, and a second input/output opticalwaveguide, wherein said microdisk and said phase shifter are bothoptically connected to said first input/output optical waveguide, andwherein said second input/output optical waveguide is opticallyconnected to said microdisk.
 14. The optical filter of claim 13, whereinsaid unit cell comprises a thin layer of semiconductor material, havingan undoped region and a doped region, in optical connection with atleast one input/output optical waveguide.
 15. The optical filter ofclaim 14, wherein said filter is a programmable delay-line filterconstructed by interconnecting a plurality of said unit cells withoptical waveguides.
 16. The optical filter of claim 15, wherein at leastsome of said unit cells are used as 2×2 optical couplers.
 17. Theoptical filter of claim 15, wherein at least some of said unit cells areused as 2×2 optical combiners.
 18. The optical filter of claim 15,wherein at least some of said unit cells are used as optical powersplitters.
 19. The optical filter of claim 15, wherein at least some ofsaid unit cells are used as recursive delay lines.
 20. The opticalfilter of claim 15, wherein at least some of said unit cells are used asoptical path switches.
 21. The optical filter of claim 15, wherein atleast some of said unit cells are used as optical phase shifters. 22.The optical filter of claim 13, wherein said plurality of unit cells areconfigured in a two-dimensional array, and wherein substantially all ofsaid unit cells are identical in composition.
 23. A programmablesignal-processing building block comprising a combination of four unitcells each described according to claim 1, wherein said building blockhas up to four optical inputs and up to four optical outputs.
 24. Theprogrammable building block of claim 23, wherein said building block canbe programmed in a manner suitable for a filter function selected fromthe group consisting of finite impulse response filter, infinite impulseresponse filter, delay line with adjustable phase shift, optical powersplitter, optical power coupler, optical switch, and recursive opticaldelay line.
 25. A method of constructing an optical signal processorcomprising a plurality of unit cells, said method comprising: (i)providing a plurality of unit cells, wherein each unit cell includes anoptical microdisk including a circular path, an optical phase shifter, afirst input/output optical waveguide, and a second input/output opticalwaveguide, wherein said microdisk and said phase shifter are opticallyconnected by means of said first input/output optical waveguide, andwherein said second input/output optical waveguide is opticallyconnected to said microdisk; and (ii) interconnecting at least some ofsaid plurality of unit cells with a plurality of optical waveguides,said plurality of optical waveguides being connected to said unit cellsby means of said first and second input/output optical waveguides insaid unit cells.
 26. The method of claim 25, further comprising the stepof providing a thin layer of semiconductor material, having an undopedregion and a doped region, in optical connection with at least oneinput/output optical waveguide, in substantially each unit cell.
 27. Themethod of claim 25, further comprising programming at least some of saidunit cells to function as 2×2 optical couplers each comprising twoinputs and two outputs, wherein said optical microdisk of said unitcells includes two coupling regions, said two coupling regions suitablyprogrammed to establish the relative amount of power coupled betweeneach of said first and second input/output waveguides and said circularpath.
 28. The method of claim 25, further comprising programming atleast some of said unit cells to function as 1×2 optical powersplitters, wherein said optical microdisk of said unit cells includes afirst and second coupling region, said first coupling region suitablyprogrammed to partially couple light between said first input/outputoptical waveguide and said circular path of said microdisk, and saidsecond coupling region suitably programmed to fully couple light betweensaid circular path and said second input/output optical waveguide. 29.The method of claim 25, further comprising programming at least some ofsaid unit cells to function as 2×1 optical power combiners, wherein saidoptical microdisk of each of said unit cells includes first and secondcoupling regions, said first coupling region suitably programmed topartially couple light from said first input/output optical waveguideinto said circular path of said microdisk, and said second couplingregion suitably programmed to fully couple light from said circular pathto said second input/output optical waveguide.
 30. The method of claim25, further comprising programming at least some of said unit cells tofunction as recursive delay lines, wherein said optical microdisk ofsaid unit cells includes first and second coupling regions, said firstcoupling region suitably programmed to couple light between said firstinput/output optical waveguide and said circular path of said microdisk,and said second coupling region suitably programmed to not couple lightbetween said circular path and said second input/output opticalwaveguide.
 31. The method of claim 25, further comprising programming atleast some of said unit cells to function as optical path switches,wherein said optical microdisk of said unit cells includes two couplingregions, said two coupling regions both suitably programmed to eitherfully couple light between said first and second input/output opticalwaveguide and said circular path of said microdisk or both suitablyprogrammed to not couple light between said first and secondinput/output optical waveguide and said circular path of said microdisk.32. The method of claim 25, further comprising programming at least someof said unit cells to function as optical phase shifters, wherein saidoptical microdisk of said unit cells includes two coupling regions, saidtwo coupling regions being suitably programmed to not couple lightbetween first and/or second input/output optical waveguide and saidcircular path of said microdisk.
 33. The method of any of claims 27-32,further comprising the step of providing a thin layer of semiconductormaterial, having an undoped region and a doped region, in opticalconnection with at least one input/output optical waveguide, insubstantially each unit cell, wherein programming is implemented byappropriately electrically energizing said undoped and doped regions ofsaid semiconductor material.